A Ground Based Augmentation System (GBAS) is used to assist aircraft during approach and landing operations. The ground station of a GBAS broadcasts pseudorange corrections and integrity information to aircraft, which helps to remove Global Navigation Satellite System (GNSS) errors impacting satellite measurements that are processed by the aircraft GNSS receivers. By using GBAS, aircraft have improved continuity, availability, and integrity performance for precision approaches, departure procedures, and terminal area operations.
A major source of error for an aircraft GNSS receiver is caused by the ionosphere, which delays GNSS signals that pass through it. The error due to ionosphere delay can almost be completely mitigated by a GBAS under nominal conditions when the ionosphere is uniform between the GBAS ground station and the aircraft GNSS receiver because the ionosphere delay for signals received by the GBAS ground station and the GNSS receiver will be similar. However, when ionosphere disturbances (e.g., ionosphere storms or other anomalous ionosphere activity) produce a non-uniform ionosphere (referred to as an ionosphere gradient), the ionosphere delay for the signals received by the GBAS ground station and the aircraft GNSS receiver can be different. This difference in ionosphere delay can cause the pseudorange corrections broadcast by the GBAS ground station and applied by the aircraft to be less accurate. When there are large distances between the GBAS ground station and the aircraft, it is possible for the variation in ionosphere delay to result in unacceptably large position errors in the aircraft navigation position solution. Mitigation of large ionosphere gradients can be accomplished via (1) cooperative mitigation between the aircraft and GBAS ground station; or (2) conservative screening based on worst case GNSS satellite geometries combined with assuming the worst case ionosphere gradient. Option 1 requires costly equipment for both the ground and airborne systems, while option 2 results in degraded system continuity and availability.
A real-time screen of all possible GNSS satellite geometries is performed, which removes measurements that may lead to unacceptable position errors in the presence of the worst case ionosphere gradient. More specifically, a GBAS could automatically assume that the worst case ionosphere gradient is always present. When a GBAS ground station checks the possible geometry configurations that an aircraft may be using, any GNSS satellite geometries that produce an error larger than a tolerable error limit, assuming the worst case ionosphere gradient is present, are broadcast to the aircraft with the indication that they should not be used.
The set of available geometries may also be restricted by inflating integrity-related parameters (e.g., broadcast sigmas) such that only usable geometries are available to the aircraft. In exemplary embodiments, this includes: (1) identifying all credible satellite geometries; (2) computing a Maximum Ionosphere Error in Vertical Position (MIEV); (3) computing the smallest possible Vertical Protection Limit (VPL) for this potentially hazardous subset of credible geometries; (4) when any geometries in this subset have a VPL less than the Vertical Alert Limit (VAL) for the desired category of precision approach, begin a search to find the smallest inflation factors that include the VPL above the VAL for all geometries in the subset of credible geometries.
One such integrity related parameter is the Vertical Ionosphere Gradient (VIG) standard deviation, referred to herein as sigma-vig (σvig). Typically, σvig is calculated for a future time based on the GNSS satellites that will be in view of the GBAS at a future time. Since GNSS satellites orbit the earth twice each sidereal day, over time, different GNSS satellites rise and set from the perspective of the GBAS. For every cycle, the calculation of σvig is performed for a subsequent epoch for all predicted GNSS satellites that will be in view of the GBAS at the future time on all predicted sub-geometries. The larger of the values between the σvig calculated for one time step in the future and the σvig value previously computed for the current time step is broadcast to the aircraft.